The present invention relates to organic semiconductors and to the use thereof in organic electronic devices.
Organic semiconductors are being developed for a number of different applications which can be ascribed to the electronics industry in the broadest sense. The structure of organic electroluminescent devices (OLEDs) in which these organic semiconductors are employed as functional materials is described, for example, in U.S. Pat. Nos. 4,539,507, 5,151,629, EP 0676461 and WO 98/27136. However, these devices still exhibit considerable problems which require urgent improvement:    1. The compounds usually used do not have a sufficiently low LUMO (lowest unoccupied molecular orbital). Compounds having a lower LUMO are required for easier electron injection and thus for a reduction in the operating voltage.    2. The colour coordinates of many blue emitters are still unsatisfactory.    3. The thermal stability, in particular of blue dopants, is inadequate.    4. The lifetime and efficiency of blue-emitting organic electroluminescent devices should be increased still further for high-quality applications.
For fluorescent OLEDs, condensed aromatic compounds, in particular anthracene or pyrene derivatives, for example 9,10-bis(2-naphthyl)anthracene (U.S. Pat. No. 5,935,721), are used, in particular, as host materials, especially for blue-emitting electroluminescent devices, in accordance with the prior art. WO 03/095445 and CN 1362464 disclose 9,10-bis(1-naphthyl)anthracene derivatives for use in OLEDs. Further anthracene derivatives are disclosed in WO 01/076323, WO 01/021729, WO 04/013073, WO 04/018588, WO 03/087023 and WO 04/018587. Host materials based on aryl-substituted pyrenes and chrysenes are disclosed in WO 04/016575. For high-quality applications, it is necessary to have improved host materials available.
Prior art which may be mentioned in the case of blue-emitting compounds is the use of arylvinylamines (for example WO 04/013073, WO 04/016575, WO 04/018587). However, these compounds are thermally unstable and cannot be evaporated without decomposition, which requires high technical complexity for OLED production and thus represents an industrial disadvantage. A further disadvantage is the emission colour of these compounds: while dark-blue emission (CIE y coordinates in the range 0.15-0.18) is described in the prior art with these compounds, it has not been possible to reproduce these colour coordinates in simple devices in accordance with the prior art. Green-blue emission is obtained here. It is not apparent how blue emission can be generated with these compounds. For high-quality applications, it is necessary to have improved emitters available, particularly in relation to device and sublimation stability.
The matrix material used in phosphorescent OLEDs is frequently 4,4′-bis(N-carbazolyl)biphenyl (CBP). The disadvantages are short lifetimes of the devices produced therewith and frequently high operating voltages, which result in low power efficiencies. In addition, CBP has an inadequately high glass transition temperature. Furthermore, it has been found that CBP is unsuitable for blue-emitting electroluminescent devices, which results in poor efficiency. In addition, the construction of the devices with CBP is complex since a hole-blocking layer and an electron-transport layer additionally have to be used, Improved triplet matrix materials, based on keto compounds, are described in WO 04/093207, but these likewise do not give satisfactory results with all triplet emitters.
The electron-transport compound used in organic electroluminescent devices is usually AlQ3 (aluminium trishydroxyquinolinate) (U.S. Pat. No. 4,539,507). This cannot be vapour-deposited in a residue-free manner since it partially decomposes at the sublimation temperature, which represents a major problem, in particular for production plants. A further disadvantage is the strong hygroscopicity of AlQ3, as is the low electron mobility, which results in higher voltages and thus in lower power efficiency. In order to prevent short circuits in the display, it would be desirable to increase the layer thickness; this is not possible with AlQ3 owing to the low charge-carrier mobility and resultant increase in voltage. The inherent colour of AlQ3 (yellow in the solid), which can result in colour shifts, especially in blue OLEDs, due to reabsorption and weak re-emission, furthermore proves unfavourable. Blue OLEDs can only be produced here with considerable efficiency and colour location deficiencies.
There thus continues to be a demand for improved materials, in particular emitting compounds, especially blue-emitting compounds, but also host materials for fluorescent and phosphorescent emitters, hole-transport materials and electron-transport materials, which are thermally stable, result in good efficiencies and at the same time in long lifetimes in organic electronic devices, give reproducible results during the production and operation of the device and are readily accessible synthetically.